there's gotta be a simpler solution but I reckon this works

=AND(LEN(C7)<8,MAX(FREQUENCY(MATCH(MID(C7,ROW(INDIRECT("1:"&LEN(C7))),1),$H$1:$N$1,0),MATCH(MID(C7,ROW(INDIRECT("1:"&LEN(C7))),1),$H$1:$N$1,0)))=1,COUNT(MATCH(MID(C7,ROW(INDIRECT("1:"&LEN(C7))),1),$H$1:$N$1,0))=LEN(C7))